Re: pgsql: Fix parallel index and index-only scans to fall back to serial.

On 14/07/18 00:56, Robert Haas wrote:
> On Fri, Jul 13, 2018 at 2:22 PM, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
>> I just bumped into this comment, from commit 09529a70bb5, and I can't make
>> sense of it:
>>
>>> + /*
>>> + * We reach here if the index only scan is not parallel,
>>> or if we're
>>> + * executing a index only scan that was intended to be
>>> parallel
>>> + * serially.
>>> + */
>>
>>
>> What was that intended to say?
>
> There are two ways that you can reach that code. One is that you have
> the thing that shows up in EXPLAIN output as "Index-Only Scan". The
> other is that you have the thing that shows up in EXPLAIN output as
> "Parallel Index-Only Scan", but you didn't get any workers, so now
> you're falling back to running what was intended to be a parallel plan
> without parallelism i.e. serially. The comment is intended to alert
> you to the fact that an intended-as-parallel scan can end up here in
> corner cases where the plan doesn't end up being parallel. We've had
> some difficulty in consistently getting that case correct.

Ah, gotcha. I changed that (and the other copies) per David's suggestion.

- Heikki